Devices of this type for automatically controlling the edges of webs of sheeting in blister machines have been known for a long time. In the production of blister packs, a forming sheet and a cover sheet are sealed together in a blister machine, wherein pockets, which are filled with the pharmaceutical products, are formed beforehand in the forming sheet. To guide the sheets, i.e., the forming sheet and the cover sheet, a web edge control system is required, because, as the sheets are being transported through the machine, they have the tendency to move transversely to the main travel direction. This tendency is caused by deviations in the parallelism of the deflecting rollers and guide rollers and also by inaccuracies in the cutting of the rolls of forming sheet or cover sheet.
To control this movement of the sheets, the forming sheet has up to now been guided transversely to the travel direction by two guide plates located at the last deflecting roller before the forming station.
When a sensor provided for this purpose sends a signal that the forming sheet threatens to leave the tolerance range, a motor pushes the guide plates transversely to the sheeting travel direction to correct the direction in which the sheeting is traveling.
Especially in the case of sheetings of aluminum foil, the use of lateral guide plates to shift the sheeting can lead to a situation in which the sheeting does not follow the movement of the guide plates. Instead, the edges of the sheeting climb up the guide plates and fall back on themselves.
This problem does not exist to the same degree in the case of sheets of PVC because of their greater stiffness, but fine dust abraded from the PVC does accumulate on the guide plates.